The magnetic properties of a permanent magnet material, such as the known Fe-Nd-B permanent magnet alloy (i.e. Nd2Fe14B), can be separated into two categories: intrinsic and extrinsic properties. Intrinsic properties can be altered by substitution of alloying elements on lattice sites. For example, in the Fe--Nd--B alloy system, the intrinsic magnetic properties can be altered by direct substitution of other elements for the iron, neodymium, or boron sites. U.S. Pat. No. 4,919,732 describes element substitutions that alter magnetic properties for Fe--Nd--B alloys made by rapid solidification using melt spinning. However, generally, enhancing one magnetic property in this manner comes at the price of decreasing another magnetic property.
The extrinsic magnetic properties can be altered by changing the alloy microstructure. For example, by rapid solidification, such as melt spinning and high pressure gas atomization, it is possible to maximize the magnetic properties by forming an extremely fine grain size directly from the melt or by over quenching and crystallizing grains during a short time anneal.
However, there is a problem of maintaining the improved magnetic properties attributable to fine grain structure following consolidation of the rapidly solidified powder or flakes to a magnet shape at high temperatures (such as employed in hot extrusion and hot isostatic pressing) for extended times. During consolidation, the high temperature involved drastically alters (degrades) the extrinsic magnetic properties of the resulting permanent magnet. This degradation defeats the magnetic property advantages achieved by the initial rapid solidification process.
The aforementioned U.S. Pat. No. 4,919,732 describes melt spinning an Nd--Fe--B melt to form rapidly solidified flakes that retain zirconium, tantalum, and/or titanium and boron in solid solution. After the melt spun flakes are comminuted to less than 60 mesh, they are subjected to a recrystallization heat treatment to precipitate diboride dispersoids to stabilize the fine grain structure. The recrystallized flakes are then comminuted to a size of 5 microns or less, cold compacted to a magnet shape under an applied magnetic field, and sintered at high temperature.
A disadvantage associated with the use of melt spinning to rapidly solidify the Nd--Fe--B melt results from the flake shaped particles produced. These particles are difficult to handle and properly consolidate to optimum magnetic properties. As described in the patent, the melt spun flakes are first comminuted to less than 60 mesh size, heat treated, and then further comminuted to less than 5 microns size prior to compaction and sintering.
A disadvantage associated with use of precipitated diborides of hafnium, zirconium, tantalum, and/or titanium to slow grain growth is the alloy competition between using the boron to form the boride and using the boron to form the 2-14-1 phase. This means that during alloying extra boron needs to be added to compensate for this effect which changes the location on the ternary Nd--Fe--B phase diagram and the resulting solidification sequence. In addition, it is found that the transition metal carbonitrides are more stable than their respective borides in the 2-14-1 type magnets. Furthermore, there is a wide range of stoichiometries found in the transition metal carbonitride precipitates. This greater variability in structure allows more freedom in selecting appropriate heat treating cycles.